Serotonin is a vaso-active amine which is highly concentrated in the blood platelet. Release of serotonin in response to platelet activating stimuli may play a role in many pathologic states. The absence of platelet serotonin is associated with severe platelet dysfunction and a hemorrhagic diathesis. We have modified a sensitive assay to accurately measure platelet endogenous serotonin content in normal subjects and patients. Young, heavy platelets contain four times this serotonin of old, light platelets. Thus, platelet serotonin content may be a useful means to measure platelet turnover rates. Isolation of age dependent platelet cohorts permits the study of serotonin uptake, content and release in a variety of pathologic states.